Method for fabricating stringers

ABSTRACT

A method for fabricating stringers is provided. The co-cured composite stringer includes a stringer and a mandrel positioned within a channel defined in the stringer. A plurality of strips are positioned within an opening of the mandrel, and a substrate layer, such as aircraft skin, is positioned adjacent to the stringer. During application of the substrate layer to the stringer, the mandrel and plurality of strips support the stringer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/830,291, filed Apr. 20, 2004 now U.S. Pat. No. 7,293,737, which ishereby incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to stringers and, more particularly, to acollapsible mandrel having reinforcing strips therein that may beemployed to co-cure stringers, as well as an associated fabricationmethod.

2) Description of Related Art

Aircraft utilize a variety of elements for reinforcement due to theapplication of bending, torsional, shear, and direct loading on theaircraft. One of the basic reinforcing structures used within aircraftinclude stringers (also termed stiffeners and longerons), whichtypically run spanwise within the wing. Stringers may be fabricated intovarious cross sections, such as I-beams, Z, and “hat” sections.Stringers are typically a lightweight composite formed into a tape orfabric, which includes using a plurality of fibers embedded in a matrixof resin or adhesive to support and bond the fibers together (i.e.,pre-impregnated or “prepreg”). A lay-up method is typically used to formthe stringers, wherein the skin, typically also formed as a prepreg, isapplied to a stringer using sufficient head pressure. Autoclaving or asimilar method is then used to cure the stringer and the aircraft skintogether.

There are a variety of mandrels used to co-cure stringers. For example,there are hollow mandrels that, when vented to an autoclave, press fromthe inside out. However, the hollow mandrels cannot support the tapehead rolling over the stringers. Another type of mandrel includes hardrubber mandrels placed between the skin and stringer, but this mandrelhas proved to be difficult to remove from long stringers followingcuring. There are also expendable mandrels that may be dissolved aftercuring, but this mandrel requires a messy and impractical dissolutionprocess. Finally, there is a re-usable elastomeric mandrel system(REMS), which is essentially a rubber shaped mandrel full of ceramicbeads. In spite of this design, the REMS is impractical to use withlonger stringer lengths.

Further, U.S. Pat. No. 5,368,807 to Lindsay discloses a method forvacuum bag molding fiber reinforced resin matrix composites withoutrequiring the use of a mandrel. The method includes laying prepregstiffener hat sections on the inside portion of a rigid portion of avacuum bag. An elastomeric tube is inserted within each of the stiffenersections, and may be inflated to conform to the shape of the stiffener.A sealing wedge is placed on the end of each of the stiffeners to holdand seal the elastomeric tube to the rigid portion of the vacuum bag.Following curing, the wedge is removed, and a vacuum drawn on theelastomeric tube is used to collapse the tube for removal from thevacuum bag.

Lindsay shows the stringers or longerons arranged linearly, but does notpermit arranging and co-curing the stringers in a variety of othercontours, as would be desirable. In addition, the rigid vacuum bag onlysupports the stringer during autoclaving but does not support thestringer during laying up of the aircraft skin. Thus, the aircraft skinin Lindsay is not capable of being applied over a recessed area of a“hat-shaped” stringer such that the elastomeric tube does not have towithstand the head pressure of tape laying aircraft skin to thestringer, where the head pressure may also cause collapse of theaircraft skin within the recess or of the stringer itself.

It would therefore be advantageous to provide a mandrel used inco-curing stringers that may withstand the pressure applied duringlay-up. Also, it would be advantageous to provide a stringer and mandrelthat may be formed into various shapes prior to co-curing. Finally, itwould be advantageous to provide a mandrel that may be easily removedfollowing co-curing.

BRIEF SUMMARY OF THE INVENTION

The invention addresses the above needs and achieves other advantages byproviding a composite stringer that may be supported by a collapsiblemandrel during lay-up and co-curing. The composite stringer isfabricated in part with the support of a mandrel and reinforcing stripsduring application and curing of the substrate layer to the stringer.The mandrel and reinforcing strips may be removed following co-curing ofthe composite stringer and reused to fabricate additional stringers.

In one embodiment, the co-cured composite stringer includes a stringerand a mandrel positioned within a channel defined in the stringer. Aplurality of strips are positioned within an opening of the mandrel, anda substrate layer, such as uncured pre-impregnated skin, is positionedadjacent to the stringer. During application of the substrate layer tothe stringer, the mandrel and plurality of strips support the stringer.

In variations of the present invention, the co-cured composite stringerincludes a plurality of strips stacked horizontally and verticallywithin the opening of the mandrel such that each of the strips may slidewith respect to one another. The plurality of strips are preferablypositioned within the opening of the mandrel such that the stripssubstantially conform to the shape of the mandrel. In addition, theplurality of strips are pliable such that the mandrel and strips arecapable of bending to a desired contour. The plurality of strips may berectangular in shape and comprise a polymeric material (e.g., nylon).The stringer may be an uncured pre-impregnated reinforcing material,such as an epoxy graphite, while the mandrel may be an elastomericmaterial. In another advantageous embodiment of the present invention,the mandrel and plurality of strips are capable of withstanding at least90 psi of head pressure.

In another aspect of the present invention, a collapsible lay-up mandrelassembly is provided that comprises a mandrel defining an openingtherethrough, and a plurality of strips slidably positioned within theopening of the mandrel.

The present invention further provides a method of fabricatingstringers. The method includes providing a stringer that defines achannel therein. A plurality of strips are positioned within a mandrel,and the mandrel is then positioned within the channel of the stringer.The method also includes applying a substrate layer to the stringer. Themandrel and plurality of strips are capable of supporting the stringerduring application of the substrate layer, which may include headpressures of at least 90 psi. Finally, the stringer and the substratelayer are attached to one another, such as by curing the stringer andsubstrate layer together.

In additional embodiments of the present invention, the method furtherincludes bending the plurality of strips and mandrel to a desiredcontour. The plurality of strips may also be slid from the mandrelfollowing application of the substrate layer. Applying a vacuum to themandrel following removal of the plurality of strips may be employed toremove the mandrel.

The present invention therefore provides a co-cured composite stringerthat may be formed into a variety of contours and lengths. An uncuredsubstrate layer is applied to an uncured stringer such that thesubstrate layer may be co-cured to the stringer. The mandrel and stripsare used to retain a desired shape of the stringer during laying of thesubstrate layer to the stringer and during co-curing. In addition, themandrel and strips may be manipulated to various configurations prior toapplication of the substrate layer, and may be removed and reusedfollowing co-curing. A variety of materials may be used for individualelements in embodiments of the present invention to fabricate co-curedstringers that provide various degrees of reinforcement to the substratelayer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a perspective view of a plurality of co-cured stringers havingstrips positioned within a respective mandrel according to oneembodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view of one of the co-curedstringers shown in FIG. 1;

FIG. 3 is a perspective view of a stringer illustrating that each stripis removable from the mandrel of one embodiment of the presentinvention;

FIG. 4 is an enlarged cross-sectional view of a stringer with the stripsremoved and vacuum applied to the mandrel of one embodiment of thepresent invention; and

FIG. 5 shows a flowchart of a method of fabricating co-cured stringersaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Referring now to the drawings and, in particular to FIG. 1, there isshown a substrate 16 having a plurality of stringers 10 arrangedthereon. The stringers 10 are capable of being arranged in a variety ofpositions, for example, longitudinally along the span of an aircraftwing or contoured to a specific curvature. Additionally, the stringers10 could be any number of dimensions to accommodate a variety ofreinforcement applications. The term “stringer” is not meant to belimiting and could be any stiffener, longeron, or reinforcing memberthat is used to provide support to another structure, such as anaircraft skin. Furthermore, the stringers 10 could be useful to supportstructures other than aircraft skins or wings, such as with marine,construction, or automotive structures, where a lightweightreinforcement is desired. Thus, the substrate 16 could be any suitablematerial that that is capable of being reinforced by the stringers 10.For purposes of example, however, reference will be made to an aircraftskin as the substrate 16.

The stringers 10 are generally fabricated by applying an aircraft skin16 to the stringer and co-curing the aircraft skin and underlyingstringer together. In order to ensure that the stringer 10 maintains itsshape during laying of the aircraft skin 16, a mandrel 12 is positionedbetween the stringer and the aircraft skin. In addition, a number ofstrips 14 are positioned within the mandrel 12 to also providereinforcement during lay-up of the aircraft skin 16. The stringers 10shown in FIGS. 1-2 are “hat” shaped as known to those skilled in theart, although various shapes could be used in additional embodiments ofthe present invention, such that various mandrel 12 shapes could also beutilized.

The composite material used for each of the stringers 10 and aircraftskin 16 is generally a lightweight material, such as an uncuredpre-impregnated reinforcing tape or fabric (i.e., “prepreg”). The tapeor fabric typically includes a plurality of fibers such as graphitefibers that are embedded within a matrix material, such as a polymer,e.g., an epoxy or phenolic. The tape or fabric could be unidirectionalor woven depending on the degree of reinforcement desired. Thus, theprepreg tape or fabric is generally laid onto a mandrel or mold topreform the tape or fabric into the desired shape of the stringer 10 asdefined by the mandrel or mold. The stringers 10 could be any suitabledimension to provide various degrees of reinforcement, and could becomprised of any number of plies of prepreg tape or fabric. Similarly,the aircraft skin 16 could be various sizes and thicknesses for bothindividual and multiple plies depending on the weight and amount ofsupport desired.

In one embodiment, each stringer 10 is preformed and generally defines achannel or recess. Further, each preformed stringer 10 is typicallyuncured. A mandrel 12 is positioned within the channel of the stringer10. The mandrel 12 has a shape that substantially conforms to thecorresponding channel defined by the stringer 10 such that the mandrelmay provide support to the stringer during subsequent lay up of theaircraft skin 16. Also, the mandrel 12 defines an opening, such as alengthwise opening, such that a plurality of longitudinally extendingstrips 14 may be inserted within the opening to provide reinforcement tothe mandrel.

The mandrel 12 of the illustrated embodiment has a trapezoidal shape toconform to a hat-shaped stringer 10, although the mandrel could have anynumber of other shapes to conform to differently shaped stringers.Similarly, the mandrel 12 could be various dimensions so long as themandrel defines an opening that enables strips 14 to be insertedtherethrough. The mandrel 12 may be formed of any elastomeric material,such as Teflon® (E.I. du Pont de Nemours and Company) coated silicone orhard rubber, but is preferably pliable to enable the mandrel to conformto various configurations. Thus, the mandrel 12 may be bent somewhat tofit within and conform with channels defined by a preformed stringer 10that may not be linear, such as curved or piecewise linear channels. Inaddition, the mandrel 12 may be fabricated with any suitable technique,such as with extrusion or casting.

As shown in FIG. 2, the strips 14 are inserted within the openingdefined in the mandrel 12. The strips 14 are advantageously pliable andmay slide with respect to one another. Because the strips 14 are pliableand individually capable of sliding axially within the mandrel 12, thestrips can have a variety of cross-sectional shapes, as shown in FIG. 1.The greater the number of strips 14, i.e., smaller cross section, themore flexible the mandrel 12 is, such that more complex contours may beachieved with a greater number of strips. The collection of strips 14substantially conform to the shape of the opening defined by the mandrel12 such that there are no gaps between the strips and the mandrel. FIG.2 demonstrates that the strips 14 may be arranged horizontally andvertically with respect to one another, as well as adjacent to eachother, although the strips could be arranged in any number ofconfigurations within the mandrel 12. Furthermore, although some of thestrips 14 are shown as being rectangular, the strips could be any numberof dimensions and cross-sectional shapes that may complement the shapeof the opening defined by the mandrel 12, as well as any number oflengths depending on the structure to be reinforced.

In the embodiment of the present invention shown in FIG. 2, there areouter strips 14 that contact the surface of the mandrel 12, whileinterior strips contact other adjacent strips but do not contact anysurface of the mandrel. The widest strips 14 in the mandrel 12, the toptwo strips shown in FIG. 2, are generally split into two separate stripsto aid in the removal of the strips from the mandrel following curing.However it is understood that the strips 14 shown in FIG. 2 could bearranged within the mandrel 12 in any desired configuration with anynumber and dimension of strips. The interior strips 14 are generallyeasier to remove and are thus removed first, although the strips couldbe removed in any order desired.

The strips 14 are preferably a polymeric material, such as nylon that iscapable of withstanding the temperatures associated with co-curing.However, the material used for the strips 14 depends on the temperatureused in co-curing to prevent deformation of the strips by heating thestrips above their melting temperature, as well as the configuration ofthe strips arranged within the mandrel 12, such that various materialscould be used in variations of the present invention in additionalembodiments. The strips 14 may have a coefficient of thermal expansionthat is higher than the surrounding stringer 10 and aircraft skin 16such that the strips expand when heated to apply additional pressureduring curing, but also contract upon cooling to aid in removal of thestrips from the mandrel 12. The strips 14 could be fabricated throughextrusion, molding, or any other suitable process.

A method of fabricating co-cured stringers 10 is illustrated in theflowchart shown in FIG. 5. After each of the stringers 10 is preformedin an uncured state, the stringers are typically first placed in atrough section of a lay-up mandrel that defines a shape corresponding tothat of the preformed stringers. The stringers 10 are uncured so thatthe prepreg may be cured at the same time that the stringer is cured tothe aircraft skin 16. Next, the mandrel 12, having the strips 14inserted within the mandrel as described above, is placed within arespective channel or recess defined by the stringers 10. The strips 14could also be inserted within the mandrel 14 after the mandrel ispositioned in the channel or recess of the stringer 10, as opposed toalready being assembled within the mandrel. As a result of theirpliability, the mandrel 12 and strips 14 could be bent or otherwisearranged within the channel or recess of the stringer 10 in any numberof configurations, such that the mandrel and strips need not becustom-made for a particular stringer shape or contour.

The aircraft skin 16 is generally applied over the mandrel 12 andexposed portions of the stringer 10 with pressure being applied, such asa head pressure of about 90 psi. The mandrel 12 and strips 14 arecapable of withstanding this head pressure to ensure that the aircraftskin 16 and stringer 10 maintain their shape and are not undesirablydeformed. The aircraft skin 16 is applied as a tape or fabric such thata sufficient amount of tape or fabric of the aircraft skin contacts thestringer 10 to allow the stringer to cure to the aircraft skin 16 duringautoclaving. Laying the aircraft skin 16 onto the stringer 10 andmandrel 12 is preferably automated, although it is understood thathand-laying techniques could also be employed in alternative embodimentsof the present invention. It is also understood that additionaltechniques, as known to those skilled in the art, could be utilized inapplying the aircraft skin 16 to the stringer 10. In this regard, in onealternative of the present invention, the mandrel 12 could be placed onthe aircraft skin 16 and prepreg applied over the mandrel such that thestringer could be formed over the mandrel and applied directly to theaircraft skin.

The stringer 10 and aircraft skin 16 are generally co-cured withautoclaving, as known to those skilled in the art, which generallyinvolves the application of heat and pressure under vacuum. It isunderstood that other techniques could be used to co-cure the stringer11 and the aircraft skin 16, such as with a vacuum bag, as known tothose skilled in the art.

Following co-curing of the stringer 10 and the aircraft skin 16, each ofthe strips 14 may be removed, as illustrated in FIG. 3. Thus, the strips14 are capable of being removed individually by hand. As discussedabove, the strips 14 that do not contact any surface of the mandrel 12are typically removed first. After each of the strips 14 have beenremoved from the mandrel 12, the mandrel is subjected to a vacuum tocollapse it slightly so that the mandrel can be removed from each of thestringers 10. The mandrel 12 is also capable of being removed from theco-cured stringer 10 by hand without the application of a vacuum. FIG. 4illustrates a cross section of a mandrel under vacuum, whichdemonstrates that the edges buckle slightly to enable easy removal ofthe mandrel. Each of the strips 14 and mandrel 12 may thus be reusedduring subsequent co-curing of additional stringers 10. Although FIG. 3illustrates the strips 14 capable of being removed by hand, it isunderstood that other techniques of removing both the strips and themandrel could be employed, such as with an automated machine.

In alternative embodiments of the present invention, the stringer 10could be pre-cured prior to being cured to the aircraft skin 16, or viceversa. In addition, both the stringer 10 and aircraft skin 16 could bepre-cured and attached together. Furthermore, it is understood that thestringers 10 could be shaped in a lay-up mandrel rather than being in anuncured and preformed shape such that the stringer is molded into a hatsection in the same lay-up mandrel that laying up of the aircraft skintakes place. Also, it is understood that the aircraft skin 16 could belaid over the “hat-shaped” side of the stringer 10 (i.e., the sideopposite to the channel or recess) in addition to over the channel orrecess and co-cured to the stringer such that the aircraft skin isco-cured to both sides of the stringer. Alternatively, the aircraft skin16 could be laid over the “hat-shaped” side of the stringer 10 only andsubsequently co-cured to the stringer. Thus, the mandrel 12 and strips14 would support the stringer 10 if the aircraft skin is applied ineither manner.

Therefore, the combination of the mandrel 12 and strips 14 providereinforcement during application of the aircraft skin 16 andautoclaving. Thus, the mandrel 12 and strips 14 prevent the stringers 10from buckling when the aircraft skin 16 is laid over the recessed areaof the stringers. The stringers 10 may be fabricated in any number ofdimensions depending on the type of support and configuration desired.In addition, the mandrel 12 and strips 14 may be advantageously bondedto the aircraft skin 16 in one autoclave cure cycle and removedfollowing co-curing and reused for additional curing.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which thisinvention pertains having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method for fabricating co-cured stringers comprising: providing astringer, the stringer defining a channel therein; positioning aplurality of strips within a mandrel such that a plurality of rows ofstrips are stacked on top of one another, wherein each row comprises aplurality of strips and each of the strips is configured to slide withrespect to one another; positioning the mandrel within the channeldefined in the stringer; applying a substrate layer to the stringer,wherein the mandrel and plurality of strips support the stringer duringthe applying of the substrate layer; and curing the stringer and thesubstrate layer such that the stringer and substrate layer are attached.2. The method according to claim 1, further comprising bending theplurality of strips and mandrel to a desired contour.
 3. The methodaccording to claim 1, wherein applying comprises applying uncuredpre-impregnated reinforcing material.
 4. The method according to claim1, further comprising removing the plurality of strips by sliding thestrips from the mandrel following curing.
 5. The method according toclaim 4, further comprising applying a vacuum to the mandrel followingremoving the plurality of strips.
 6. The method according to claim 5,further comprising removing the mandrel following applying the vacuum.7. The method according to claim 1, wherein positioning comprisessliding the plurality of strips within the mandrel.
 8. A method forfabricating stringers comprising: providing a stringer, the stringerdefining a channel therein; sliding a plurality of strips within amandrel such that a plurality of rows of strips are stacked on top ofone another, wherein each row comprises a plurality of strips and eachof the strips is configured to slide with respect to one another;positioning the mandrel within the channel defined in the stringer;applying a substrate layer to the stringer, wherein the mandrel andplurality of strips support the stringer during the applying of thesubstrate layer; and attaching the stringer and the substrate layertogether.
 9. The method according to claim 8, wherein attachingcomprises curing the stringer and the substrate layer such that thestringer and substrate layer are attached.
 10. The method according toclaim 8, further comprising bending the plurality of strips and mandrelto a desired contour.
 11. The method according to claim 8, whereinapplying comprises applying the substrate layer at a head pressure of atleast 90 psi.
 12. The method according to claim 11, further comprisingremoving the plurality of strips by sliding the strips from the mandrelfollowing attaching the stringer and the substrate layer together. 13.The method according to claim 12, further comprising applying a vacuumto the mandrel following removing the plurality of strips.
 14. Themethod according to claim 13, further comprising removing the mandrelfollowing applying the vacuum.
 15. A method for fabricating stringerscomprising: providing a stringer, the stringer defining a channeltherein; positioning a plurality of strips within a mandrel such that aplurality of rows of strips are stacked on top of one another, whereineach row comprises a plurality of strips and each of the strips isconfigured to slide with respect to one another; positioning the mandrelwithin the channel defined in the stringer; applying an aircraft skin tothe stringer, wherein the mandrel and plurality of strips support thestringer during the applying of the aircraft skin; and attaching thestringer and the aircraft skin together.
 16. The method according toclaim 15, wherein attaching comprises curing the stringer and theaircraft skin such that the stringer and aircraft skin are attached. 17.The method according to claim 15, further comprising bending theplurality of strips and mandrel to a desired contour.
 18. The methodaccording to claim 15, wherein applying comprises applying uncuredpre-impregnated reinforcing material.
 19. The method according to claim15, further comprising removing the plurality of strips by sliding thestrips from the mandrel following attaching.
 20. The method according toclaim 15, wherein positioning comprises sliding the plurality of stripswithin the mandrel.
 21. The method according to claim 5, whereinapplying a vacuum comprises applying a vacuum to the mandrel such thatthe mandrel returns to its original shape upon removal of the vacuum.22. The method according to claim 1, wherein positioning comprisespositioning a plurality of strips within a mandrel such that the stripsare arranged in a plurality of horizontal rows stacked vertically on topof one another.
 23. The method of according to claim 1, wherein applyingcomprises laying up the substrate layer onto the stringer while applyingpressure.
 24. The method according to claim 1, wherein positioningcomprises positioning the plurality of strips within the mandrel suchthat a plurality of outer strips are positioned in contact with themandrel and at least one inner strip does not contact any surface of themandrel.
 25. The method according to claim 1, wherein positioningcomprises positioning the plurality of strips within a longitudinalopening defined in the mandrel such that the plurality of stripssubstantially fill and conform to the longitudinal opening.